Given its most general meaning, a mechanical “bearing” is a machine element that constrains relative motion between moving parts to allow only a desired motion. As examples, a bearing might allow linear motion of a moving part or a free rotation around a fixed axis. Many bearings also have means to facilitate the desired motion by minimizing friction.
Bearings may be classified according to the type of operation, the motions allowed, or the directions of the loads (forces) applied to the parts. There are many types of bearings, with varying shapes, materials, lubrications, and principles of operation.
If classified by principle of operation, the simplest type of bearing is a plain bearing. A plain bearing may be simply a cylindrical bearing surface of a bore with a shaft passing through it, or of a planar surface that bears another. In the absence of lubricant, the opposing bearing surfaces are in contact, and the friction force is influenced by the tribological properties of the materials. With the presence of a fluid lubricant (e.g., oil), relative motion of the bearing surfaces causes the lubricant to shear, which generates hydrodynamic pressure that, with sufficient relative velocity, allows a thin film of the lubricant to support the force between the shaft and bearing (i.e., they are no longer in contact). The lubricant may also be a solid film (e.g., graphite), and the shearing of the solid lubricant also prevents the two bearing surfaces from being in contact.
Other common types of bearings are rolling-element bearings, magnetic bearings, and various types of fluid bearings. With rolling-element bearings, the load is carried by a ring of balls or by rollers. With a magnetic bearing, the load is carried by a magnetic field. Fluid bearings can be categorized as either hydrostatic or hydrodynamic. Hydrostatic fluid bearings use an externally pressurized gas or liquid flowing into the thin film between bearing surfaces to support the load. Hydrodynamic fluid bearings, or so-called self-acting bearings, develop the load-bearing pressure within the gas or liquid film through the relative motion of the bearing surfaces. Some fluid bearings may also rely on a combination of hydrostatic and hydrodynamic pressure development. Fluid bearings can be in a wide range of configurations from plain bearings to tilting pad bearings to foil bearings.
FIGS. 1 and 2 are a perspective view and a front view, respectively, of a rotating shaft having a conventional foil gas bearing. FIG. 3 is a cross-sectional view along section line 3-3 of FIG. 2.
A shaft 11 rotates freely in a supporting sleeve 13. Sleeve 13 typically has a cylindrical inner surface, but other geometries are possible, such as a variable radius with multiple lobes.
A top foil layer 15 is supported by a “bump” foil layer 16. The bump foil layer 16 is the compliant layer. Other foil bearing designs have been devised which incorporate different compliant structures, such as leaf-type springs, metal meshes, and elastomeric elements, and those which incorporate multiple, separate top foils and various foil retention features. FIGS. 1-3 and 5-8 depict a bump foil structure with a single top foil for the purposes of example without excluding the applicability of the invention, described below, to all types of foil gas bearings.